Replace variables with their corresponding values in source code chunk

Question

I'd like to show parameter values and not params$... in R Markdown output. For example, the first code chunk below displays params$file in the output, but I'd like to replace that with samples.txt. I tried adding a second chunk with message, but that outputs a white code chunk and I'd like a gray background like all other R code blocks.

---
output: html_document
params:
   file: samples.txt
---

```{r read, message=FALSE, collapse=TRUE, comment=""}
x <- read_tsv(params$file)
x
```
This just needs a gray background
```{r print, echo=2, collapse=TRUE, comment=""}
message('x <- read_tsv("', params$file, '")')
x
```

Answer 1

You could modify the source hook. A solution tailored to your need follows immediately. For a more general approach, that replaces all elements in params, scroll down.

---
output:
  pdf_document: default
  html_document: default
params:
  file: samples.txt
---


```{r, include=FALSE}
library(knitr)
library(stringr)
default_source_hook <- knit_hooks$get('source')

knit_hooks$set(source = function(x, options) {
  x <- str_replace_all(x, pattern = 'params\\$file', paste0("'",params$file,"'"))
  default_source_hook(x, options)
})
```

```{r print, echo=T, comment="", eval = T}
print(params$file)
```

First we save the default hook that would be used depending on the output file type (render_html or render_latex etc.). Then we change the source hook: we replace all occurrences of params$file with its value and then throw the source code back into the default hook we saved before.

In this case this results in:

This magic works, because we only modify the source code that will be printed, not the one being evaluated!

---

Update: A more general Approach

I played a bit with your example and created a more general hook. It should replace all elements of the form params$... in your code chunks. It even checks for the type of value and adds quotes if it is a character value.

Check the following MRE:

---
output:
  pdf_document: default
  html_document: default
params:
  file: samples.csv
  age: 28
  awesome: true
  34: badname
  _x: badname
---


```{r, include=FALSE}
library(knitr)
library(gsubfn)

default_source_hook <- knit_hooks$get('source')

knit_hooks$set(source = function(x, options) {
  x <- gsubfn(x = x, pattern = "params\\$`?([\\w_]+)`?", function(y) {
    y <- get(y, params)
    ifelse(is.character(y), paste0("'", y, "'"), y)
  })
  default_source_hook(x, options)
})
```

```{r print, echo=T, comment="", eval = T}
file <- params$file
age  <- params$age
awsm <- params$awesome
# dont name your variables like that! works though...
badexmpls <- c(params$`34`, params$`_x`)
```

We make use of gsubfn(). This function allows us to use a function for the replacement attribute (not possible in common gsub). This function takes on the elements found, but, thanks to regex only the part after the $. So in this chunk, y equals file, age and awesome.